Transparent substrate coated with at least one thin layer

ABSTRACT

A transparent glass substrate ( 1, 1′, 1 ″) coated with one or more thin layers ( 6, 16, 28 ) is disclosed, wherein the thin layer most distant from the glass substrate is a composition comprising silicon nitride, carbonitride, oxynitride or oxycarbonitride, said thin layer most distant from the glass substrate being covered by a second layer ( 7, 17 ) which protects against high-temperature corrosion.

The invention relates to transparent substrates, especially glasssubstrates, that are provided with at least one thin layer.

The main application of the invention is the manufacture of so-calledfunctional windows used in the building industry or for equippingvehicles. Hereafter, “functional” window should be understood to mean awindow in which at least one of the substrates is coated with thinlayers intended to give it special properties, especially thermal,electrical, optical or even mechanical properties, such as ascratch-resistant property for example.

The thin layers of greatest interest in the invention are those intendedto confer thermal properties, i.e. those which can act, especially, byreflecting long-wavelength infrared and/or solar radiation.

Thus, so-called low-emissivity layers are known, especially thin silverlayers, or layers of a doped metal oxide of the F:SnO₂ or ITO type,filtering layers having a solar-protection function, for example thosebased on metal layers of the nickel-chromium-alloy type, thicker silverlayers or TiN-type metal nitride layers.

Stacks may provide one or more of these layers which will be denotedbelow by the term “functional layers”. These layers are usually combinedwith other layers to form a stack, for various reasons.

Thus, it is usually intended to combine them with at least one coatingof dielectric material, which coating(s) lies(lie) above the functionallayer and/or are inserted between the carrier substrate and thefunctional layer. This is firstly for optical reasons: these coatings,chosen so as to have an appropriate refractive index and thickness,allow the visual appearance of the window, especially in reflection, tobe adjusted in an interferential manner. Furthermore, above thefunctional layer, they may also provide protection from chemical ormechanical attack. Mention. may also be made of Patents EP-544,577,EP-573,325 and EP-648,196 which describe stacks using an F:SnO₂-typefunctional layer combined with another layer of a dielectric layer ofthe SiO₂, SiOC or SiON type, Patents EP-638,528, EP-745,569 andEP-678,484 which describe stacks using one or more silver layersalternating with one or more layers of a dielectric of the metal-oxidetype, or Patent EP-650,938 which use a TiN-based layer combined with twooxide layers or patent EP-511,901 which uses a layer of the nitridednickel-chromium type between two particular metal oxides.

More recently, it has also been sought to provide these layers ofdielectric material with the function of protecting the functionallayers during high-temperature heat treatments, of thebending/toughening type, of their glass substrates. Materials based onsilicon nitride have come to be very useful, especially from this pointof view: optically, they have a refractive index close to 2 andtherefore similar to those of most of the metal oxides normally used asdielectric layers, for example of the SnO₂ type. However, they also actas a barrier to atmospheric oxygen with respect to the functional layer,thus preserving it from any deterioration of the high-temperatureoxidation type. Furthermore, they are “inert” with respect to oxygen athigh temperature in the sense that their properties, especially theiroptical properties, remain unchanged after a heat treatment of thebending/toughening type. Thus, it is possible to design stacks in whichthe functional layer is covered by an Si₃N₄ layer optionally combinedwith other layers, with thermal and optical properties which remainidentical after bending/toughening: this is the teaching, for example,of patent EP-0,718,250 which describes stacks of the glass/oxidelayer(s)/silver/metal/oxide layer(s)/Si₃N₄ type, the outermost layerbeing made of Si₃N₄.

However, it has proved to be the case that even with this type of stack,industrial yields were not optimal in the sense that still too high anumber of coated substrates had to be scrapped, once they had been bentor toughened, because of the appearance of pinhole-type optical defectsvisible to the naked eye.

The object of the invention is therefore to overcome this drawback,especially by providing a novel type of stack comprising a functionallayer or layers which can undergo heat treatments, while exhibitingbetter optical quality, or at the very least optical quality which ismore reproducible and more controlled.

The subject of the invention is a transparent substrate of the glasssubstrate type coated with a thin layer based on silicon nitride,carbonitride, oxynitride and/or oxycarbonitride (hereafter denoted bythe term “silicon nitride layer”) or with a stack of thin layers, thelast of which is this silicon nitride layer. In order to preventdeterioration of this layer during heat treatments of the bending ortoughening type, especially in contact with an atmosphere containingcorrosive species of the Na₂O type and, optionally, chlorides orsulphides, it is covered by a layer which protects against this type ofhigh-temperature corrosion and/or is “doped” by introducing at least onemetal into its composition. (The term “doped” should not be taken herein its sense known in electronics, rather it indicates that the nitridelayer exhibits properties, most particularly properties of resistance tohigh-temperature corrosion, which are enhanced by the presence of this(these) metallic additive(s).

In fact, it has turned out that the silicon nitride layer completelyfulfilled its role of protecting the subjacent layers fromhigh-temperature oxidation. However, on the other hand, under certainconditions encountered in heat treatments of the toughening type, butabove all of the bending type, this silicon nitride layer could besusceptible to deterioration not by oxidizing attack but rather byattack from chemical species present in the atmosphere in which thebending operation is carried out and are “aggressive” at hightemperature and/or which “migrate” from the second glass substrate inthe case in which the bending of several of these glass substrates iscarried out simultaneously, the substrates being superposed on a ringmould with the multilayer stack of. one of the substrates in contactwith the other glass substrate. At least one type of these species hasbeen identified: these are all alkali metal compounds of the sodiumtype, especially Na₂O vapour. This high-temperature sensitivity hashitherto resulted in the pinholes mentioned above, surface attack of thesilicon nitride propagating into the rest of the stack.

The invention has therefore consisted in keeping silicon nitride for itsvery useful oxygen-barrier properties but improving its high-temperaturedurability by two combined or alternative means:

according to the first variant, it is “sheathed” by a protective layerwhich is not intended to block oxygen but which will block the Na₂O-typecorrosive species, by forming a perfectly impermeable barrier, either bybeing chemically inert with respect to these corrosive species or byhaving a good affinity with them so as to filter them out by absorbingthem;

according to the second variant, the nitride is chemically modified inorder to make it more resistant but without, however, making it lose itsoxygen-barrier properties.

This highly effective approach makes it possible for the scrap rate whenbending the glass substrate to be very significantly reduced, byconsiderably limiting the appearance of the optical defects observedhitherto. This approach is completely unexpected in that silicon nitrideis normally regarded as quite a durable material from a mechanicalstandpoint, and chemically rather inert. It might have been expectedthat the origin of the optical defects detected in the stack would bethe layers which do not have such a degree of durability and aresubjacent to the silicon nitride layer, for example the functionallayers or the first layer of the stack, the one in direct contact withglass. On the contrary, the inventors have therefore shown thatcorrosion propagated via the final nitride layer.

The invention allows a compromise to be made, namely to keep siliconnitride despite this demonstrable weakness, by improving it.

According to the first variant, the silicon nitride is thereforeprotected by an overlayer.

In a first embodiment, this overlayer is placed on the silicon nitridelayer (preferably directly but optionally via at least one other layerof the dielectric type) either in the form of a metal or in the form ofa metal oxide which is substoichiometric in terms of oxygen. This layeris intended to be completely oxidized during the heat treatment. Duringthe deposition, before the treatment, it preferably has a geometricalthickness of at most 10 nm, especially of between 1 and 5 nm. In fact,there will be oxidation, and therefore modification of the opticalproperties of the stack after heat treatment, essentially resulting inan increase in the light transmission. However, these modifications arenot of extreme importance since the protective layer is preferablyconfined within very small thicknesses, although these are sufficient tofulfil its function. They are, in any case, under complete control sincethe properties of the protective layer are chosen so that it completelyoxidizes during the heat treatment.

According to the second embodiment of this first variant, the protectivelayer is deposited on top of the silicon nitride layer (directly orindirectly, as in the previous embodiment) in the form of a metal oxide,oxycarbide and/or oxynitride, especially to a geometrical thickness ofat most 20 nm, especially of between 2 nm and 10 nm. According to thissecond embodiment, no modification in the properties, especially theoptical properties of the stack is observed after heat treatment.

The protective layer, which may or may not undergo oxidation during theheat treatment, comprises at least one metal whose oxide is capable ofblocking/absorbing/filtering the species corrosive at high temperatureother than oxygen, especially of the Na₂O type. This metal is preferablychosen from niobium Nb, tin Sn, tantalum Ta, titanium Ti and zirconiumZr. Niobium is particularly advantageous, its oxide having a highaffinity with alkali metals of the sodium type.

According to the second variant of the invention, the silicon nitridelayer is “doped” by introducing up to 25% by weight, especially between3. and 12% by weight, of at least one metal. This metal is preferablyaluminium.

The silicon-nitride-based layer may advantageously form part of athin-layer stack and may especially be in the stack above a functionallayer having thermal properties, directly or via at least one other thindielectric or metal layer. This functional layer may in particular be afiltering, solar-protection, selective, low-emissivity and/orelectrically conductive layer.

The stack may comprise one or more functional layers, which may or maynot be of the same nature, for example two or three functional layers.

The functional layer may be of the metallic type, especially one basedon silver, gold, aluminium, nickel, chromium, optionally, a nitride, orstainless steel. It is possible to have not a single functional layer,but at least two functional layers separated by at least one dielectriccoating.

These functional layers, their thicknesses and their optical performancecharacteristics are especially described in the aforementioned patents.Mention may more particularly be made to the above Patent EP-0,718,250which uses one or more silver layers in a stack which is completed by asilicon nitride layer merely to give the stack“bendability”/“toughenability”. Mention may also be more particularlymade of Patent Application EP-0,847,965 which discloses stacks of thetype having two silver layers, these being designed for the purpose ofimproving their bendability or toughenability, and also making use of atleast one silicon nitride layer: the invention allows the quality of thestacks described in these two patents to be further improved.

Thus, we have stacks which are, diagrammatically, of the type: silverlayer placed, on the one hand, between an “inner” dielectric coating (onthe carrier substrate side) and, on the other hand, and “outer”dielectric layer preferably via a thin metal layer, the said “outer”dielectric coating comprising the improved silicon nitride layeraccording to the invention. In the case of a stack consisting of twosilver layers alternating with three dielectric coatings, at least thedielectric layer “outermost” with respect to the carrier substrate iscompleted by the improved silicon nitride layer according to theinvention.

The functional layer may also be of the metal-nitride type, especiallyone based on TiN, CrN, NbN or ZrN.

The functional layer may also be of the doped-metal-oxide type, such asITO, F:SnO₂, In:ZnO, F:ZnO, Al:ZnO or Sn:ZnO.

The invention preferably relates to thin layers deposited by vacuumtechniques, especially the technique of sputtering, optionally enhancedby a magnetic field. This is a well-controlled technique for depositingmetal or metal oxide layers or metal nitride or silicon nitride layers.In the latter case, metal or silicon targets are used in suitablereactive atmospheres with gases of the O₂ or N₂ type. The invention alsorelates to thin layers deposited by other techniques, especially thoseof the type comprising pyrolysis directly on the ribbon of float glass,pyrolysis of a powder or CVD (Chemical Vapour Deposition), thesetechniques being suitable for depositing optionally doped metal oxidelayers, for depositing metal nitride layers, as described in PatentEP-638,527 and EP-650,938, and for depositing layers based on siliconnitride optionally also comprising. oxygen and/or carbon, as describedin Patent Application FR 97/01468 of 10 Feb. 1997.

The invention also relates to stacks of thin layers, some of whichlayers, for example, the first layers, may be deposited by pyrolysis,and others, especially the next layers, using a vacuum technique, in asubsequent operation.

The subject of the invention is also the application of the substratecoated according to the invention to the manufacture of windows whichexploit the electrical properties of the functional layer(s) of thestack as heated windows, as well as its application to the manufactureof windows having the above-mentioned thermal properties, which windowsare furthermore bendable/toughenable.

The invention relates to the use of these windows both in the buildingindustry and as windows for fitting into vehicles of the motor-vehicletype, these having a “monolithic” structure (a single rigid substrate),being laminated with two rigid glass substrates or being asymmetricallylaminated (one glass substrate combined with at least one sheet ofpolymer that absorbs mechanical energy and a so-called sheet of polymerof the self-healing type, these both generally being based onpolyurethane, as described for example in Patent EP-673,757). Mentionmay especially be made of vehicle windscreens and side windows.

The invention is advantageous irrespective of the bending orbending/toughening technique envisaged. The following may, in anon-exhaustive manner, be mentioned:

the technique of bending glass substrates running over a curved-profileshaping bed, consisting especially of straight or curved rotating rolls,as is described in Patents EP-133,114, EP-263,030, EP-474,531 andEP-593,363;

the technique of gravity bending, in which a glass substrate or twosuperposed glass substrates are placed horizontally on peripheralbending formers mounted on carriages which move through a reheatfurnace, as described in Patents EP-317,409, EP-465,308, EP-640,569 andWO 97/23420 and particularly adapted to the manufacture of laminatedwindows; and

the technique of bending which involves a step of pressing and/orsuction against an upper solid bending former associated with a lowerannular bending former, as described in Patents EP-324,690, EP-438,342,EP-665,822, EP-459,898, EP-578,542 and EP-660,809.

A toughening operation, especially a thermal toughening operation, maycomplete these bending operations or may replace them. In all cases,this requires reheating the glasses up to at least 500° C. and generallyabout 550 to 620° C., which temperatures exacerbate the corrosive effectof certain kinds of vapour of the Na₂O type, from which observations theinvention stems.

The invention will be described in detail below with the aid ofnon-limiting examples illustrated by figures:

FIG. 1: a glass substrate coated with a stack of thin layers, includinga silver layer;

FIGS. 2 and 3: a glass substrate coated with a stack of thin layers,including two silver layers; and

FIG. 4: a laminated window incorporating the substrate according to oneof FIGS. 1, 2 and 3, once it has been bent.

These figures are extremely diagrammatic and do not respect thethickness proportions between the various materials shown, in order tomake them easier to understand.

EXAMPLE 1

This relates to a glass substrate 1 shown in section in FIG. 1, namely aflat 2 mm thick substrate made of silica-soda-lime float glass on whichhas been deposited a stack of the type described in Patent EP-0,718,250,improved according to the invention, namely the stack:

glass⁽¹⁾/Si₃N₄ ⁽²⁾/ZnO⁽³⁾/Ag⁽⁴⁾/Nb⁽⁵⁾/Si₃N₄ ⁽⁶⁾/Nb⁽⁷⁾

All the layers are deposited by sputtering.

The deposition apparatus comprises at least one sputtering chamberprovided with cathodes equipped with targets made of suitable materialsbelow which the substrate 1 passes in succession. The recommendeddeposition conditions for each of the layers for this example are:

the silver layer 4 is deposited using a silver target in an argonatmosphere;

the silicon-nitride-based layers 2 and 6 are deposited by reactivesputtering in a nitrogen atmosphere using a target made of silicon dopedwith 1% boron;

the ZnO layer 3 is deposited by reactive sputtering in an argon/oxygenatmosphere, containing approximately 40% oxygen by volume, using a zinctarget;

the Nb layer 5 for protecting the silver layer is deposited bysputtering in an inert argon atmosphere, using an Nb target; and

the layer 7 according to the invention, intended to protect thesubjacent Si₃N₄ layer labelled 6, is deposited under the same conditionsas the Nb layer 5.

The power densities and run speeds of the substrate are adjusted in aknown manner in order to obtain the desired layer thicknesses.

Table 1 below gives the nature of the layers, and their thicknesses innanometres, of the stack in Example 1:

TABLE 1 EXAMPLE 1 Si₃N₄ (2) 20 ZnO (3) 20 Ag (4) 10 Nb (5) 1 Si₃N₄ (6)40 Nb (7) 2

EXAMPLE 2

This relates to a glass substrate 1′ shown in section in FIG. 2, thissubstrate being identical to the previous substrate 1, on which has beendeposited a stack which includes two silver layers and may be of thetype of those described in Patent EP-0,847,965 mentioned previously andimproved according to the invention, namely the stack:

glass^((1′))/SnO₂ ⁽⁸⁾/ZnO⁽⁹⁾/Ag⁽¹⁰⁾/Nb⁽¹¹⁾/Si₃N₄⁽¹²⁾/ZnO⁽¹³⁾/Ag⁽¹⁴⁾/Nb⁽¹⁵⁾/Si₃N₄ ⁽¹⁶⁾/Nb⁽¹⁷⁾

The layers are deposited under the same conditions as in Example 1. Inthis case, the SnO₂ layer (8) is deposited by sputtering in a reactiveatmosphere containing oxygen, using a tin target.

Table 2 below gives the nature of the layers and their thicknesses innanometres:

TABLE 2 EXAMPLE 2 glass (1′) — SnO₂ (8) 20 ZnO (9) 17 Ag (10) 9 Nb (11)0.7 Si₃N₄ (12) 65 ZnO (13) 25 Ag (14) 9 Nb (15) 0.7 Si₃N₄ (16) 37.5 Nb(17) 2

EXAMPLE 3

This relates to a glass substrate 1″ shown in section in FIG. 3, thesubstrate being identical to the previous substrates 1 and 1′, on whichhas been deposited a stack which includes two silver layers and is ofthe type of those described in Patent EP-0,847,965 improved according tothe invention.

The stack is as follows: glass^((1″))/SnO₂⁽¹⁸⁾/ZnO⁽¹⁹⁾/Ag⁽²⁰⁾/Ti⁽²¹⁾/ZnO⁽²²⁾/Si₃N₄⁽²³⁾/ZnO⁽²⁴⁾/Ag⁽²⁵⁾/Ti⁽²⁶⁾/ZnO⁽²⁷⁾/Si₃N₄ ⁽²⁸⁾ doped with 7% Al

The layers are deposited under the same conditions as those relating toExamples 1 and 2.

The 7% aluminium-doped Si₃N₄ layer (28) is deposited by reactivesputtering in a nitrogen atmosphere, using a target made of an Si/Alalloy.

Table 3 below gives the nature of the layers and their thicknesses innanometres:

TABLE 3 EXAMPLE 3 glass (1″) — SnO₂ (18) 17 ZnO (19) 17 Ag (20) 9 Ti(21) 1 ZnO (22) 10 Si₃N₄ (23) 55 ZnO (24) 20 Ag (25) 9 Ti (26) 1 ZnO(27) 10 Si₃N₄ (28) 25 doped with 7% Al

Each of the three coated substrates according to Examples 1, 2 and 3then undergoes gravity bending in a ring mould mounted on a movablecarriage, which moves through a reheat furnace. Superposed on it, in thering mould, is a second glass substrate 29 which is identical to thesubstrates 1, 1′ and 1″ but not coated with layers. The stack of layersis on the upper face of the substrate 1, 1′ or 1″, and is therefore incontact with the lower face of the second substrate (“in contact” doesnot necessarily mean in continuous contact, it being possible for thereto be air trapped at the interface between the two substrates). Once thebending operation has been carried out, the stack of layers therefore ison the concave face of the first substrate 1, 1′ or 1″. The twosuperposed substrates are then separated and then, again in a knownmanner, joined together via an approximately 0.8 mm thick sheet 30 ofpolyvinyl butyral in order to form a laminated window, shown in FIG. 4,which can be used as a windscreen.

Conventionally, the faces of the glass-substrates 1 and 29 are numberedstarting from the face that is intended to face the outside once it hasbeen fitted into the vehicle. In this case, we have a concave, face 2multilayer stack.

Alternatively, the stack in the laminate may also advantageously be aface 3 stack on a concave face. In this case, the bending operation iscarried out so that it is the substrate 1, 1′ with the layers which isin the ring mould on top of the layer-free substrate 29, with themultilayer stack in contact with the upper face of the layer-freesubstrate 29.

The substrates of Examples 1, 2 and 3 were examined before and afterbending and then after laminating.

The conclusions are as follows:

after bending, the light transmission of the substrates 1 and 1′increases, which means that the final Nb protective layer“encapsulating” the subjacent Si₃N₄ layer has completely oxidized;

compared with substrates which do not contain this final Nb layer, theoptical quality is superior, pinholes no longer, or almost no longer,appear and the thermal performance is also maintained;

the light transmission of the substrate 1′ does not change; and

the laminated windows obtained meet all the required criteria for beingused as a windscreen.

It should be noted that, as an alternative to the final Nb layer, it isalso possible within the scope of the invention to deposit a thin layerof tin, zirconium or titanium (or to deposit Nb₂O₅, SnO₂, ZrO₂ or TiO₂layers directly). These metals, most especially Nb, Sn and Ti, all have,in fact, the common property of forming, by oxidizing, a compound withsodium so as to limit its diffusion into the subjacent layers.

1. A transparent glass substrate (1, 1′, 1″) coated with one or morethin layers (6, 16, 28), wherein the thin layer most distant from theglass substrate is a composition comprising silicon nitride, said thinlayer most distant from the glass substrate being covered by a secondlayer (7, 17) which protects against high-temperature corrosion, whereinthe composition comprising silicon nitride forms part of a stack of thinlayers and is deposited on top of at least one functional layer (4, 10,14, 20 and 25) having thermal properties or havingelectrically-conductive properties which comprises a metal, ametal-nitride, a doped-metal-oxide, or mixtures thereof.
 2. Thesubstrate according to claim 1, wherein the second layer (7, 17) isdeposited in the form of a metal or in the form of a metal oxide whichis substoichiometric in terms of oxygen, and is intended to becompletely oxidized during a subsequent heat treatment.
 3. The substrateaccording to claim 2, wherein the second layer has a thickness of atmost 10 nm.
 4. The substrate according to claim 1, wherein the secondlayer (7, 17) is a metal oxycarbide or oxynitride.
 5. The substrateaccording to claim 4, wherein the second layer has a thickness of atmost 20 nm.
 6. The substrate according to claim 1, wherein the secondlayer (7, 17) comprises at least one metal whose oxide is capable ofblocking or absorbing corrosive species comprising Na₂0 at hightemperature.
 7. The substrate according to claim 1, wherein saidfunctional layer (4, 10, 14, 20, 25) comprises a metal.
 9. The substrateaccording to claim 1, wherein said functional layer (4, 10, 14, 20, 25)comprises a metal-nitride.
 10. The substrate according to claim 1,wherein said functional layer (4, 10, 14, 20, 25) comprises adoped-metal-oxide.
 11. The substrate according to claim 7, wherein thesubstrate is provided with a stack of thin layers comprising at leastone silver-based layer (4) deposited, between an inner dielectriccoating (2, 3) and an outer dielectric coating (6) wherein said outerdielectric coating comprises the silicon nitride layer (6).
 12. Thesubstrate according to claim 1, wherein the stack of thin layerscomprises at least two functional layers.
 13. The substrate according toclaim 1, wherein the second layer is effective to protect againsthigh-temperature corrosion to prevent deterioration of the thin layermost distant from the glass substrate during heat treatments for bendingor toughening in an atmosphere containing corrosive species
 14. Thesubstrate according to claim 6, wherein the metal is selected from thegroup consisting of Nb, Sn, Ta, Ti, Zr, and mixtures thereof.
 15. Thesubstrate according to claim 7, wherein the metal is selected from thegroup consisting of silver, gold, aluminum, nickel, chromium, stainlesssteel, and mixtures thereof.
 16. The substrate according to claim 8,wherein the metal-nitride is selected from the group consisting of TiN,CrN, NbN, ZrN, and mixtures thereof.
 17. The substrate according toclaim 9 wherein the doped-metal-oxide is selected from the groupconsisting of ITO, F:Sn02, In:ZnO, F:ZnO, Al:ZnO, Sn:ZnO, and mixturesthereof.
 18. The substrate according to claim 3, wherein the secondlayer has a thickness of between 1 and 5 nm.
 19. The substrate accordingto claim 5, wherein the layer (7, 17) deposited in the form of a metaloxide, oxycarbide or oxynitride has a thickness of between 2 and 10 nm.20. The substrate according to claim 1, wherein the thin layer mostdistant from the glass substrate is a composition consisting essentiallyof silicon carbonitride.
 21. The substrate according to claim 1, whereinthe thin layer most distant from the glass substrate is a compositionconsisting essentially of silicon oxynitride.
 22. The substrateaccording to claim 1, wherein the thin layer most distant from the glasssubstrate is a composition consisting essentially of siliconoxycarbonitride.